The treads of modern tires must meet performance standards which require a broad range of desirable properties. Rubber compositions suitable for tire treads should exhibit not only desirable strength and elongation, particularly at high temperatures, but also good cracking resistance, good abrasion resistance, desirable skid resistance and low tan delta values at low frequencies for desirable rolling resistance. Additionally, a high complex dynamic modulus is necessary for maneuverability and steering control.
Major tire manufacturers have developed tread compounds which provide lower rolling resistance for improved fuel economy and better skid/traction for a safer ride. Most passenger tread compounds contain either solution styrene butadiene rubber (S-SBR) or emulsion styrene butadiene rubber (E-SBR) blended with either high cis polybutadiene (cis BR) or high vinyl content polybutadiene (HV-BR). Three types of performance are important in tread compounds. They include good wear resistance, good traction and low rolling resistance. Therefore, it is important to select the appropriate SBR to develop a tire tread with good overall performance.
Compounds which provide good wet traction should have high tangent delta values at low temperatures (0.degree. C.), while compounds having low rolling resistance should have low tangent delta values at high temperatures (50.degree.-100.degree. C.). These two properties are very difficult to obtain from an E-SBR/Cis BR blended tire compound since wet skid/traction and low rolling resistance are contradictory to each other. For example, low glass transition high cis polybutadiene rubber has very poor wet skid/traction, while it provided lower rolling resistance and good wear resistance. High styrene content E-SBR has a high glass transition temperature which results in high tangent delta at 0.degree. C. and 50.degree.-100.degree. C. respectively. It provides higher rolling resistance, good wet traction and fast wear.
It has been reported that wear is related with the styrene content of SBR in a tread compound. High styrene SBR is often used in high performance tread to improve skid/traction and provide a better handling and safer tire. This performance comes at the expense of rolling resistance and tread wear properties.
International Patent Application WO 92/20737 describes a tire tread composition comprising i) a first rubber component comprising about 5-15 parts of nitrile-butadiene rubber per hundred parts of rubber (phr); ii) a second rubber component comprising about 20-95 phr of styrene butadiene rubber; and iii) a third rubber component comprising 0-47 phr of high cis polybutadiene rubber. U.S. Pat. No. 4,894,420 describes tire treads having cis 1,4-polyisoprene, at least one copolymer containing acrylonitrile rubber, and a rubber based on polybutadiene. U.S. Pat. No. 4,791,178 describes a tire tread composition of at least 30% of a conjugated diene-monovinyl aromatic hydrocarbon copolymer. U.S. Pat. No. 4,866,131 describes a rubber composition for use in tires containing a high styrene SBR. U.S. Pat. No. 4,616,685 describes tire treads containing a plasticizing ester, a styrene-butadiene rubber having more than 25% styrene by weight, and a second rubber ingredient chosen from butyl rubber, butyl halide rubber, butadiene/acrylonitrile copolymer, or mixtures thereof. U.S. Pat. No. 4,433,094 describes tire treads having a particular type of carbon black, styrene-butadiene rubber having a specific bound styrene content, and optionally another diene rubber. U.S. Pat. No. 4,788,241 describes a curing system comprised of a curative, a paraffin wax and pentaerythritol tetrastearate, which is said to cure rubbers such as natural rubber, cis-polyisoprene, polybutadiene, solution and emulsion poly(styrene-butadiene), EPDM, poly(acrylonitrile-butadiene), and mixtures thereof. Japanese Patent No. JP 63270751 A and JP 63270751 A2 describe tire tread compositions having abrasion resistance and good traction, comprising SBR containing 25-60% styrene, nitrile rubber, carbon black, silica and silane couplers.
The purpose of this invention is to provide a tire tread composition which has excellent wet traction and low rolling resistance.